WASHINGTON — Just how rich is Mitt Romney? Add up the wealth of the last eight presidents, from Richard Nixon to Barack Obama. Then double that number. Now you’re in Romney territory.

He would be among the richest presidents in American history if elected — probably in the top four.

He couldn’t top George Washington who, with nearly 60,000 acres and more than 300 slaves, is considered the big daddy of presidential wealth. After that, it gets complicated, depending how you rate Thomas Jefferson’s plantation, Herbert Hoover’s millions from mining or John F. Kennedy’s share of the vast family fortune, as well as the finer points of factors like inflation adjustment.

But it’s safe to say the Roosevelts had nothing on Romney, and the Bushes are nowhere close.

The former Massachusetts governor has disclosed only the broad outlines of his wealth, putting it somewhere from $190 million to $250 million. That easily could make him 50 times richer than Obama, who falls in the still-impressive-to-most-of-us range of $2.2 million to $7.5 million.

“I think it’s almost hard to conceptualize what $250 million means,” said Shamus Khan, a Columbia University sociologist who studies the wealthy. “People say Romney made $50,000 a day while not working last year. What do you do with all that money? I can’t even imagine spending it. Well, maybe …”

Of course, an unbelievable boatload of bucks is just one way to think of Romney’s net worth, and the 44 U.S. presidents make up a pretty small pond for him to swim in. Put alongside America’s 400 or so billionaires, Romney wouldn’t make a ripple.

So here’s a look where Romney’s riches rank — among the most flush Americans, the White House contenders, and the rest of us:

—Within the 1 percent:

“Romney is small potatoes compared with the ultra-wealthy,” said Jeffrey Winters, a political scientist at Northwestern University who studies the nation’s elites.

After all, even in the rarefied world of the top 1 percent, there’s a big difference between life at the top and at the bottom.

A household needs to bring in roughly $400,000 per year to make the cut. Romney and his wife, Ann, have been making 50 times that — more than $20 million a year. In 2009, only 8,274 federal tax filers had income above $10 million. Romney is solidly within that elite 0.006 percent of all U.S. taxpayers.

Congress is flush with millionaires. Only a few are in the Romney realm, including Rep. Darrell Issa, R-Calif., and Massachusetts Sen. John Kerry, who was the Democratic presidential nominee in 2004. Kerry’s ranking would climb much higher if the fortune of his wife, Teresa Heinz, were counted. She is the widow of Sen. John Heinz, heir to the Heinz ketchup fortune.

Further up the ladder, top hedge fund managers can pocket $1 billion or more in a single year.

At the top of the wealth pile sits Bill Gates, worth $59 billion, according to Forbes magazine’s estimates.

—As a potential president:

Romney clearly stands out here. America’s super rich generally don’t jockey to live in the White House. A few have toyed with the idea, most notably New York Mayor Michael Bloomberg, whom Forbes ranks as the 12th richest American, worth $19.5 billion. A lesser billionaire, Ross Perot, bankrolled his own third-party campaigns in 1992 and 1996.

Many presidents weren’t particularly well-off, especially 19th century leaders such as Abraham Lincoln, James Buchanan and Ulysses S. Grant. Nor was the 33rd president, Harry Truman.

“These things ebb and flow,” said sociologist Khan. “It’s not the case that all presidents were always rich.”

A few former chief executives died in debt, including Thomas Jefferson, ranked in a Forbes study as the third-wealthiest president.

Comparing the landlocked wealth of early Americans such as Washington, Jefferson and James Madison, with today’s millionaires is tricky, even setting aside the lack of documentation and economic changes over two centuries.

Research by 24/7 Wall St., a news and analysis website, estimated Washington’s wealth at the equivalent of $525 million in 2010 dollars.

Yet Washington had to borrow money to pay for his trip to New York for his inauguration in 1789, according to Dennis Pogue, vice president for preservation at Mount Vernon, Washington’s Virginia estate. His money was tied up in land, reaping only a modest cash income after farm expenses.

“He was a wealthy guy, there’s no doubt about it,” Pogue said, and probably among the dozen richest Virginians of his time. But, “the wealthiest person in America then was nothing in comparison to what these folks are today.”

—How does Romney stand next to a regular Joe?

He’s roughly 1,800 times richer.

The typical U.S. household was worth $120,300 in 2007, according to the Census Bureau’s most recent data, although that number is sure to have dropped since the recession. A typical family’s income is $50,000.

Calculations from 24/7 Wall St. of the peak lifetime wealth (or peak so far) of Nixon, Gerald Ford, Jimmy Carter, Ronald Reagan, George H.W. Bush, Bill Clinton, George W. Bush and Obama add up to a total $118 million — while Romney reports assets of up to $250 million.

If you consider only those presidents’ assets while in office, without millions earned later from speeches and books, their combined total would be substantially lower, and Romney’s riches would leave the pack even further behind.

The 10 richest presidents according to Forbes magazine, which considered estimates of their wealth while in office and the national economy of their times. Forbes says Republican primary candidate Mitt Romney would rank fourth if elected: